Sensors are used to preset a fuel-air mixture for operation of an internal combustion engine by determining the oxygen concentration in the exhaust gas of the engine. The fuel-air mixture may be in the rich range, i.e., the fuel is present in stoichiometric excess, so that only a small amount of oxygen is present in the exhaust gas in comparison with other partially unburned components. In the lean range, where more oxygen than air is present in the fuel-air mixture, the oxygen concentration in the exhaust gas is high accordingly.
Lambda probes are known for determining the oxygen concentration in the exhaust gas, detecting a lambda value&gt;1 in the lean range or &lt;1 in the rich range and lambda=1 in the stoichiometric range. In a known way, a Nernst measurement cell of the sensor supplies a detection voltage which is sent to a circuit arrangement. The detection voltage is determined here by a difference in oxygen concentration at an electrode exposed to the gas for measurement and at an electrode of the Nernst measurement cell exposed to a reference gas. The detection voltage increases or decreases according to the oxygen concentration in the exhaust gas. A solid electrolyte body which is conductive for oxygen ions is arranged between the electrodes of the Nernst measurement cell.
Such sensors must be heated to temperatures above approximately 300.degree. C. in the active range in order to achieve the necessary ion conductivity of the solid electrolyte. To achieve an increase in measurement accuracy of the sensor, it is known that the operating temperature of the sensor can be controlled and regulated as necessary. It is known, in addition, that a heating device may be provided for the sensor and can be turned on or off in accordance with an operating temperature measured by the sensor.
To determine the operating temperature, it is known that an alternating voltage can be applied to the Nernst measurement cell and an a.c. resistance of the sensor can be determined with a measurement device.
A disadvantage of the known method is that the temperature-dependent a.c. resistance is determined by starting with a constant a.c. resistance of the electrodes, the solid electrolyte and the leads to the electrodes. The leads here have approximately 50% of the total resistance of the Nernst measurement cell in the operating state. Due to a manufacturing scattering, the lead resistance is subject to a relatively great scattering, so the measurement device determining the a.c. resistance of the Nernst measurement cell has an error corresponding to this scattering. The measurement device adds this scattering error to a temperature-induced fluctuation in the a.c. resistance and supplies a corresponding faulty control signal for the heating device of the sensor. This regulates the sensor at an incorrect operating temperature.